The α-β phase transition in volcanic cristobalite

Damby, D.E.; Llewellin, E.W.; Horwell, C.J.; Williamson, B.J.; Najorka, J.; Cressey, G.; Carpenter, M.A.

Abstract

Cristobalite is a common mineral in volcanic ash produced from dome-forming eruptions. Assessment of the respiratory hazard posed by volcanic ash requires understanding the nature of the cristobalite it contains. Volcanic cristobalite contains coupled substitutions of Al3+ and Na+ for Si4+; similar co-substitutions in synthetic cristobalite are known to modify the crystal structure, affecting the stability of the and forms and the observed transition between them. Here, for the first time, the dynamics and energy changes associated with the – phase transition in volcanic cristobalite are investigated using X-ray powder diffraction with simultaneous in situ heating and differential scanning calorimetry. At ambient temperature, volcanic cristobalite exists in the form and has a larger cell volume than synthetic -cristobalite; as a result, its diffraction pattern sits between ICDD - and -cristobalite library patterns, which could cause ambiguity in phase identification. On heating from ambient temperature, volcanic cristobalite exhibits a lower degree of thermal expansion than synthetic cristobalite, and it also has a lower – transition temperature (473 K) compared with synthetic cristobalite (upwards of 543 K); these observations are discussed in relation to the presence of Al3+ and Na+ defects. The transition shows a stable and reproducible hysteresis loop with and phases coexisting through the transition, suggesting that discrete crystals in the sample have different transition temperatures.

Citation

Damby, D., Llewellin, E., Horwell, C., Williamson, B., Najorka, J., Cressey, G., & Carpenter, M. (2014). The α-β phase transition in volcanic cristobalite. Journal of Applied Crystallography, 47(4), 1205-1215. https://doi.org/10.1107/s160057671401070x

• Journal Article Type: Article
• Acceptance Date: May 9, 2014
• Online Publication Date: Jun 14, 2014
• Publication Date: Aug 1, 2014
• Deposit Date: May 30, 2014
• Publicly Available Date: Jun 17, 2014
• Journal: Journal of Applied Crystallography
• Print ISSN: 0021-8898
• Electronic ISSN: 1600-5767
• Publisher: International Union of Crystallography
• Peer Reviewed: Peer Reviewed
• Volume: 47
• Issue: 4
• Pages: 1205-1215
• DOI: https://doi.org/10.1107/s160057671401070x
• Keywords: Cristobalite, Phase transitions.
• Public URL: https://durham-repository.worktribe.com/output/1430194